The number of echoes combined into a single image or image set in multiple echo imaging sequences or echo-train techniques such as rapid acquisition with relaxation enhancement (RARE), fast-spin echo (FSE), and turbo spin-echo (TSE). In RARE imaging, the ETL typically equals the acceleration factor.
As for tooth pain, it is caused by a reaction of the nerves inside a tooth's pulp chamber with the severity dependent upon the type and degree of the stimulus. What follows are some examples of symptoms you may be feeling and their possible causes.
At first it is mildly sensitive to cold. Then it becomes progressively more sensitive to cold. It may become so that the toothache pain is even spontaneous. The final stage before the pulp of the tooth dies is that the tooth becomes sensitive to heat and cold actually relieves the pain. This is a case of acute pulpitis. The pulp of the tooth is nearly dead, and will need a root canal. Click here to read more about root canal treatment.